This invention relates to the efficient, low cost production of high purity ammonium sulfate and calcium carbonate from gypsum. The gypsum may be from any source, whether mined from natural deposits or produced synthetically by a variety of means including that obtained as a byproduct from flue gas desulfurization (FGD gypsum) systems located at electric power plants, the manufacture of phosphoric acid (phosphogypsum), the production of nitrogen-phosphorus-potassium (NPK) fertilizers (nitrogypsum), the production of titanium dioxide by the sulfate process (titanogypsum), the manufacture of purified citric acid (citrogypsum), and the production of hydrofluoric acid (fluoroanhydrite (an anhydrous form of gypsum)).
While synthetic gypsum provides a cheap source of gypsum as byproducts, the physical structure of the gypsum crystalline particles may have characteristics that make it difficult to react with other chemicals. The process of the present invention overcomes this problem and other difficulties to result in an economically viable method to produce ammonium sulfate and calcium carbonate of high purity and yield and further resulting in selectable polymorphs of calcium carbonate including the crystal structure of vaterite and calcium carbonate crystals down to nano proportions. Further, the process provides for multiple concurrent calcium carbonate crystal sizes and crystal shapes to be produced simultaneously.
FGD gypsum is a synthetic product resulting from sulfur dioxide (SO2) gas emission control systems used at fossil fuel and particularly, coal combustion power plants to remove sulfur from the combustion gases using “scrubber” devices. The sulfur dioxide is derived from sulfur containing compounds in the fuels. A wet scrubber uses lime (calcium oxide or calcium hydroxide) or more typically, limestone (calcium carbonate) to react with sulfur dioxide gas to remove the sulfur in a solid form. The reaction in wet scrubbing uses a limestone (CaCO3)—water slurry to produce calcium sulfite (CaSO3) according to the following chemical reaction:CaCO3(solid)+SO2(gas)→CaSO3(solid)+CO2(gas)
To partially offset the cost of the FGD installation, the CaSO3 (calcium sulfite) may be further oxidized (known as forced oxidation) to produce CaSO4.2H2O (FGD gypsum) according to the following chemical reaction:CaSO3(solid)+½H2O(liquid)+½O2(gas)→CaSO4.½H2O(solid)Hydration CaSO4.½H2O+1½H2O→CaSO4.2H2O
Large quantities of phosphoric acid are used in the production of fertilizers and detergents. Phosphoric acid is obtained by processes based on the decomposition of phosphate minerals, such as phosphate rock (e.g. apatite or phosphorite) with an acid, such as sulfuric acid or nitric acid. Phosphogypsum is a byproduct of producing phosphoric acid by treating phosphate rock with an acid. Often, the phosphate rock contains one or more radioactive elements such as uranium and thorium and these elements may be present in the phosphogypsum.
The phosphate rock is typically treated with sulfuric acid according to the following reaction:Ca3(PO4)2+3H2SO4→2H3PO4+3CaSO4Hydration CaSO4.½H2O+1½H2O→CaSO4.2H2O
Tri-calcium phosphate reacts with sulfuric acid to form phosphoric acid and calcium sulfate (gypsum).
Phosphate rock may also be treated with nitric acid to yield phosphoric acid and calcium nitrate. The calcium nitrate is then reacted with ammonium sulfate to yield ammonium nitrate and gypsum as shown in the following reactions:Ca10F2(PO4)6+20HNO3→6H3PO4+10Ca(NO3)2+2HF(NH4)2SO4+Ca(NO3)2+2H2O→2NH4NO3+CaSO4.2H2O
Whether the phosphate rock is treated with sulfuric acid, nitric acid or another acid, the less soluble gypsum can be separated from the product phosphoric acid by filtration. The resulting phosphogypsum is usually a hemihydrate or a dihydrate, depending on process parameters and reactant concentrations.
Titanium dioxide is an important white pigment which is manufactured in large quantities wherein about half is produced by the sulfate process. The sulfate process results in byproduct calcium sulfate (titanogypsum).
Large quantities of citric acid are used in the food, pharmaceutical and detergent industries which is produced by mycological fermentation of crude sugar solutions such as molasses. In order to eliminate impurities from the citric acid, such as proteins and sugars, it is precipitated with lime (calcium oxide) to form calcium citrate. Pure citric acid is produced by acidification with sulfuric acid and byproduct calcium sulfate (citrogypsum) is removed.
In the production of hydrofluoric acid the mineral fluorspar or fluorite is heated with sulfuric acid. This results in the production of hydrofluoric acid and calcium sulfate in the anhydrous form of anhydrite (fluoroanhydrite).
Synthetically produced gypsum results from precipitation processes and consists of small, fine, crystalline particles that are chemically nearly identical to mined natural gypsum, but physically may be of smaller particle size and have different crystal structure.
For example, FGD gypsum produced at different power plants may differ slightly in chemical composition and in crystalline structure. Most chemical differences are due to impurities from the employed fuel. Structurally, however, some FGD gypsum may be composed of crystalline particles that have less surface area and are thus less reactive than other FGD gypsum crystalline particles. FGD crystals that are thicker and more spherical have less reactive surface area. Thus, a process that employs FGD gypsum as a starting material, must be able to accommodate the less reactive FGD gypsum particles as well as FGD gypsum crystalline particles resulting from other FGD installations that produce particles having flatter, more disc like structure with greater reactive surface area.
The above discussion of the relation of crystal size and shape to reactivity is also true for other types of gypsum, such as, for example, phosphogypsum, titanogypsum, citrogypsum and fluoroanhydrite.
A further consideration of the process that results in the production of synthetic gypsum is the purity of the reactants and process operations which affect purity of the gypsum byproduct.
The process of the present invention employs a chemical reaction of FGD gypsum with ammonium carbonate ((NH4)2CO3) to produce ammonium sulfate ((NH4)2SO4) and calcium carbonate (CaCO3). Both the ammonium sulfate and calcium carbonate products are commercially valuable materials and are produced by the present process in high purity and high yield.
Ammonium sulfate (21-0-0-24S) is used most commonly as a chemical fertilizer for alkaline soils. When applied to damp soil, an ammonium ion is released which creates a small amount of acid, that lowers the pH balance of the soil. In the soil, the ammonium ions are converted to nitrate by soil bacteria which contributes nitrogen to the soil and aids in plant growth. Ammonium sulfate dissolves relatively slowly (ammonium sulfate—74.4 g/100 mL at 20° C. (68° F.), urea—107.9 g/100 mL at 20° C., ammonium nitrate—150 g/100 mL at 20° C.), which makes for more efficient use and thus reduces cost compared to some other artificial fertilizers. For example, the relatively slow aqueous dissolution of ammonium sulfate affords a slow release fertilizer providing environmental benefits of less runoff of fertilizer unused by plants to streams and less leaching of fertilizer unused by plants to groundwater.
Common nitrogen fertilizers include anhydrous ammonia (82% N), urea (46% N), urea and ammonium nitrate solutions (28-32% N), ammonium sulfate (21% N) and ammonium nitrate (34% N). Ammonium sulfate (21%) is a nitrogen source with little or no surface volatilization loss when applied to most soils. It is easy to store and is not as hygroscopic as ammonium nitrate. Ammonium sulfate is a good source of sulfur when it is needed to correct or prevent a sulfur deficiency. In areas with high pH soils, the sulfur in ammonium sulfate helps lower soil pH levels.
In addition to use as fertilizer, ammonium sulfate is used as an agricultural spray adjuvant for water soluble insecticides, herbicides and fungicides. In this capacity, it functions to bind iron and calcium cations that are present in both well water and plant cells. It is particularly effective as an adjuvant for 2,4-D (amine), glyphosate, and glufosinate herbicides.
Ammonium sulfate is used in flame retardant materials because it lowers the combustion temperature and increases the production of residues or chars.
In biochemistry, ammonium sulfate precipitation is a common method for purifying proteins by precipitation. As such, ammonium sulfate is also listed as an ingredient in many vaccines used in the United States. The DTap vaccine, which protects children from diphtheria, tetanus, and whooping cough, uses ammonium sulfate for this purpose.
Fine calcium carbonate results as precipitated particles from the process of the present invention and is useful in many industries.
High purity calcium carbonate is used as dietary calcium supplement to help ensure healthy bones and teeth. Calcium carbonate supplement is effective to treat certain medical disorders related to calcium deficiency such as osteoporosis and to reduce acid in the stomach and relieve indigestion and heartburn. For irritable bowel syndrome, a calcium carbonate supplement may be taken to reduce or relieve diarrhea. Calcium carbonate is used in the production of toothpaste and as an inert substance in pharmaceutical or dietary supplement tablets.
Fine calcium carbonate is the most preferred mineral in the paper industry, used for filling and coating paper. It helps in production of the best quality printing papers. Precipitated calcium carbonate is used as a filler in paper because it is cheaper than wood fiber wherein printing and writing paper can contain 10-20% calcium carbonate. In North America, calcium carbonate has begun to replace kaolin in the production of glossy paper. Europe has been practicing this as alkaline or acid-free papermaking for several decades. Precipitated calcium carbonate is especially useful compared to ground calcium carbonate because of having a very fine and controlled particle size, on the order of 2 micrometers in diameter, which is of particular utility in producing coatings for paper.
The calcium carbonate produced by the process of the present invention has been characterized as having a high proportion of the vaterite polymorph. Calcite, aragonite, and vaterite are the three anhydrous polymorphs of calcium carbonate, in order of decreasing stability. Among the polymorph modifications of calcium carbonate, the metastable vaterite is the most practically important. Vaterite particles are applied in regenerative medicine, drug delivery and a broad range of personal care products. Vaterite-type calcium carbonate particles have unique properties such as high hydrophilicity, large surface areas, and hierarchical structures consisting of primary vaterite particles in comparison with calcite or aragonite-type polymorphs. Synthesized and natural calcium carbonates have been widely used as fillers, pigments and other functional materials for paper, foods, cosmetics, medical materials and commodities produced at industrial level. Of the three polymorphs of calcium carbonate, vaterite-type calcium carbonate particles are meta-stable, and have secondary spheres consisting of primary particles approximately 100 nm in diameter. Thus, vaterite particles have large specific surface areas with porous structures and are more hydrophilic than other two polymorphs. These properties of vaterite particles are significant as coatings for high-grade ink-jet papers, because rapid absorption of extremely small and water-based ink droplets into the coated layer without spreading to the in-plane direction is the most significant for photograde ink-jet printing.
In the oil industry, calcium carbonate is added to drilling fluids as a formation-bridging and filter cake sealing agent and can also be used as a weighting material to increase the density of drilling fluids to control the down-hole pressure.
The process of the present invention further employs a chemical reaction of gypsum resulting from acid treatment of phosphate rock with ammonium carbonate ((NH4)2CO3) to produce ammonium sulfate ((NH4)2SO4) and calcium carbonate (CaCO3). Both the ammonium sulfate and calcium carbonate products are commercially valuable materials and are produced by the present process in high purity, depending upon the composition of the phosphate rock, and high yield.